The assassination of radical al-Qaida cleric Anwar al-Awlaki, a U.S. citizen, has revived a debate about the limits of the Obama administration's authority to hunt and kill terrorists abroad.

President Barack Obama decision to escalate targeted strikes on extremists in Yemen and Somalia has already raised thorny constitutional issues, but the legal picture is further clouded in al-Awklaki's case because he was an American citizen who was born in New Mexico and emigrated to Yemen at the age of seven. It is the first publicly confirmed case of the government killing a U.S. citizen as part of the war on terror.

The Washington Post reported in 2010 that after Sept. 11, 2001 President George W. Bush authorized the CIA and later the military to assassinate U.S. citizens if strong evidence existed that an American was involved in organizing or carrying out terrorist actions against the United States or U.S. interests. As a result, Obama compiled a list of U.S. citizens who could be killed without being formally charged or tried.

It doesn't really change anything from the standpoint of whether we can target them, a senior Obama administration official told the Post. They are then part of the enemy.

Under those criteria, al-Awlaki was a legitimate target. He was an influential cleric who reportedly had a significant role in motivating a number of terrorist plots, including Faisal Shahzad's attempt to detonate a car bomb in Times Square, Umar Abdulmutallab's attempt to blow up a Detroit-bound plane and Maj. Nidal Malik Hasan's deadly shooting rampage at a military base in Fort Hood, Texas. Obama said that the al-Awlaki had taken the lead role in planning and directing the efforts to murder innocent Americans.

Al-Awlaki's father sued Obama to try and preclude the killing, arguing that it deprived Al-Awlaki of his constitutional right to due process. The lawsuit probed the boundaries of executive power, and the lawyer arguing on behalf of the Justice Department argued that courts cannot impose constraints, including judicial review, that would have the president looking over his shoulder as he weighed military action in combatting terror.

We're going to the very core powers of the President as commander-in-chief, Justice Department Attorney Douglas Letter said during the trial, which resulted in a federal judge throwing out the lawsuit.

After 9/11, Congress granted the President power to use all necessary and appropriate force against al-Qaida or those who harbored the terrorist network, and State Department Legal Adviser Harold Koh has argued that the United States is engaged in an ongoing armed conflict that eliminates the need to provide targets with legal process before the state may use lethal force.

Some have suggested that the very act of targeting a particular leader of an enemy force in an armed conflict must violate the laws of war, Koh said in a speech to the American Society of International Law. But individuals who are part of such an armed group are belligerents and, therefore, lawful targets under international law.

Critics worry that Al-Awlaki's killing could open the door for more assassinations of U.S. citizens ordered outside the sphere of traditional judicial procedure or constitutional rights. Some have questioned the evidence that Al-Awlaki was directly involved in planning plots in addition to inciting violence through sermons.

He is a midlevel religious functionary who happens to have American citizenship and speak English, Gregory Johnsen, a doctoral candidate in Near Eastern studies at Princeton, wrote in an op-ed in The New York Times. This makes him a propaganda threat, but not one whose elimination would do anything to limit the reach of the Qaida branch.

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